The synchronous digital hierarchy (SDH) standard prescribes the following predetermined transmission rates: 51.84 Mbit/s (base rate), 155.52 Mbit/s, 622.08 Mbit/s, etc. All of the prescribed transmission rates are whole multiples of the base rate.
The G.703 recommendation issued by the CCITT committee of the International Telecommunication Union (ITU) prescribes the electrical and physical characteristics of the hierarchy digital interfaces to be used for interconnecting components of digital networks which conform to the SDH standard. In particular, recommendation G.703 prescribes the type of data coding to be used for each transmission rate. For example, for 155.52 Mbit/s transmission/receiving interfaces, coded mark inversion (CMI) coding should be used. These interfaces are also known as bidirectional or transceiver interfaces.
CMI coding is a coding with two levels, A1<A2, in which a binary 0 is encoded to have the two levels A1 and A2 in succession, each for a time equal to half of the bit-time. A binary 1 is encoded by one of the two levels A1, A2 which is maintained throughout the bit-time. The two levels A1, A2 are alternated for successive binary is. The encoded CMI signal is therefore characterized in that, in the middle of the bit-time, there are no transitions or there are transitions with leading edges. Conversely, at the beginning of the bit-time, there may be either upward or downward transitions.
In general, in data-transmission networks there is a need to synchronize a component of the network with a data flow coming from a remote unit. This need arises, for example, in interfaces which are associated with digital circuits for processing data received and/or to be transmitted and which, typically, operate on data which is encoded differently. For example, the data may be coded in accordance with non-return-to-zero (NRZ) coding.
During receiving, the interface therefore has to receive a signal containing CMI-encoded data from a remote analog interface by a transmission/receiving channel formed, for example, by a pair of coaxial cables. The interface must also recognize the data, convert it into NRZ, and supply it to the digital circuits for processing. During transmission, the interface receives NRZ-encoded data from the digital processing circuits, recognizes the data, converts it into CMI, and provides the data on the transmission/receiving channel.
Timing detectors are used for synchronizing a component of the transmission network, such as an interface of the type described above for a flow of data arriving from a remote unit, for example. Due to the characteristics of CMI coding which, as stated, also has transitions in the middle of the bit-time, known timing detectors require local clock signals with a frequency of twice the frequency of the flow of data arriving, i.e., twice the data rate, to be able to produce the two transitions within the bit-time which are typical of CMI coding. In the example of a 155.52 Mbit/s data flow corresponding to a bit-time of 6.43 ns, the local clock signals have a frequency of 311.04 MHz.